The dream of a machine that can take off vertically with the safety and efficiency of a helicopter, transition to wing borne, high speed flight and then return to rotary wing flight for vertical landing, has been around for as long as flight itself.
Helicopters are versatile aircraft that are capable of efficient vertical flight from unprepared locations, but they are limited in forward speed.
Aeroplanes can carry large payloads, are capable of high speed, efficient, long range and high altitude flight but are limited operationally to prepared surfaces and facilities requiring large areas of land.
The attempt to combine these two types of flight has led to many attempts by many inventors since the first aircraft flew in the early 1900's. A lot of advances were made in this field of research during the 1950's and 1960's, the key developments being summarised at www.vtol.org on the Vertical TakeOff and Landing (VTOL) Wheel.
Of all the machines that have been built and tested there are only a small number of VTOL types that have made it into production. There are the military jet VTOL aircraft like the Harrier and the Tilt Rotor Aircraft like the V22 Osprey, but none are a true blend of the helicopter and the aeroplane. The McDonnell Douglas Helicopter (Now Boeing) Canard Rotor Wing (CRW) aircraft is a recent example of an attempt to solve this problem.
The primary issue with all attempts to convert a rotorcraft (rotary wing aircraft) from rotary wing flight to fixed wing flight with the rotor stopped to act as a fixed wing is the asymmetric lift on the rotor blades generated by the forward motion of the aircraft. This lift asymmetry results in instability as the rotor is slowed to be stopped.
Many different design configurations have attempted to push rotorcraft performance beyond the limits imposed by conventional rotors. As any rotary wing aircraft starts to move forward, the asymmetric difference in airflow between the advancing blade and the retreating blade causes imbalances in lift which results in instability. For conventional helicopters, this asymmetry also limits their maximum speed.
Conventionally designed rotorcraft, with a rotor or rotors on top of a fuselage have pushed past the pure helicopter speed limitations with the addition of wings for lift augmentation, and, or an ability to produce forward thrust from a propulsion device that is separate from the rotor, thrust augmentation. These configurations are known as Compound Helicopters and allow the rotor to be relieved from the requirements to produce both lift and thrust, therefore allowing them to be unloaded as airspeed increases. Unloading the rotor has two main advantages; it allows slower RPM on the rotor and allows a reduction in the angle of attack on the rotor blades. The results of these changes mean a delay in the onset of transonic airflow on the advancing rotor blade and an increase in the margin before blade stall is encountered on the retreating blade, both of which mean the aircraft can fly faster before these issues again become limiting factors. The advantages of these configurations are outweighed by their design complexity, high total drag and power consumption which are far higher than a comparable fixed wing aircraft. None of these aircraft have ever moved beyond the prototype phase.
Tilt Rotors are unique machines which combine some of the capabilities of a helicopter and an aeroplane in the one machine. They are less efficient in the vertical flight role than a helicopter due to their high rotor loading, and are limited to turboprop aircraft speeds in forward flight. To achieve the full benefits and capabilities of both helicopters and fixed wing aircraft is a single machine requires an aircraft known as a Stop Rotor Aircraft.
A Stop Rotor Aircraft is capable of taking advantage of the efficient vertical flight characteristics of a helicopter as well as the high speed, high altitude and long range capability of a fixed wing aircraft. A Stop Rotor Aircraft is able to attain speeds and altitudes not possible with other rotorcraft like Autogiros, Helicopters, Compound Helicopters, Gyrodynes and Tilt-Rotors.
The prior art consists of two types of Stop Rotor Aircraft proposals, the conventionally designed machines, where the rotor's rotation axis is mounted substantially at 90 degrees to the longitudinal axis of the fuselage and the tail or nose sitters where the rotor's rotation axis is parallel to the longitudinal axis of the fuselage. In this context, conventionally designed machines are those with a conventionally designed fuselage that allows the machine to takeoff and land with its longitudinal axis substantially parallel with the earth's surface using wheels, skids, floats or skis.
Tail or nose sitters are machines designed in such a way that they are required to have their fuselage in a vertical orientation during takeoff and landing when operating in their rotary wing flight mode.
There have also been other design concepts proposed like retractable, foldable and stowed rotors, but none of these have flown successfully.
Two Types and Three Methods
Stop Rotor Aircraft prior art can be divided into two different configurations using three separate methods for achieving transition between flight modes. The two configurations include firstly the conventionally designed machines where the rotor's rotation axis is mounted at substantially 90 degrees to the longitudinal axis of the fuselage and secondly the unconventional design known as Tail or Nose Sitters with the rotors rotation axis aligned with the fuselage longitudinal axis. Typically conventionally designed machines are built to perform radial airflow conversions, with the airflow acting parallel to the rotor disc, and nose or tail sitter designed machines are built to perform axial airflow conversions, with the airflow acting parallel to the rotational axis of the rotor system.
The first method of conversion is proposed by machines built to perform radial airflow conversions where the airflow acts parallel to the rotor disc. These are conventionally designed machines where the rotor's rotation axis is mounted at substantially 90 degrees to the longitudinal axis of the fuselage. The proposal is to accelerate the aircraft to wing borne flight where the non-rotating wings support the aircraft allowing the rotor to be slowed then stopped and locked in place which then allows flight to continue as a fixed wing aircraft. The reverse procedure is then applied to convert back to rotary wing flight. The rotor is then powered up again for vertical landing. These aircraft propose to be able to operate in their conversion profile for extended periods of time.
These conventionally designed aircraft have evolved from earlier compound rotorcraft concepts which are capable of operating with slowed rotors, examples being the 1950's Fairy Rotodyne, the CarterCopter, and the DARPA Heli-Plane concepts. Recent attempts to produce a stopped rotor aircraft are the Sikorsky S-72 Rotor Systems Research Aircraft (RSRA) the X-Wing and the Boeing X50A Canard Rotor Wing (CRW).
The major problem encountered by all helicopters and designs attempting to stop or start a rotor system with a radial airflow is asymmetric lift. This is applicable to any rotor system operating with a relative airflow operating parallel to the rotor disc. As the rotor is slowed, the imbalance between each side starts to increase; eventually the airflow reverses over the full span of the retreating blade. The instability created by the once per revolution directional change in airflow creates significant instability that has prevented this concept from working successfully.
The second method of conversion is proposed by machines built to perform axial airflow conversions where with the airflow acts parallel to the rotational axis of the rotor system. These are unconventional designs known as Tail or Nose Sitters. These designs are built with the rotors rotation axis aligned with the fuselage longitudinal axis. As the name suggests, tail or nose sitters are required, due to their construction, to takeoff and land with their longitudinal axis pointing substantially vertically toward the sky or ground when in rotary wing modes of operation. Although it may be possible for this configuration to perform a conventional takeoff and landing in fixed wing mode, it cannot perform a rolling takeoff or landing in rotary wing modes of operation. These aircraft are able to operate in their conversion profile for extended periods of time.
There have been a number of rotorcraft design solutions proposed using this configuration, examples being the German Focke-Wulf Triebflügel from World War II and more recently the Thorpe SEEOP Spin Wing. The Thorpe SEEOP Spin Wing proof of concept prototype is the only Stop Rotor Aircraft that has successfully changed from rotary to fixed wing flight and back again. This aircraft proposed the use of large efficient rotors, but this limits the functionality of its fuselage space due to the rotor drive system being inside the fuselage.
The third method of conversion is performed by machines during a transient window of opportunity at very low or zero airspeed. These aircraft are conventionally designed machines, where the rotor's rotation axis is mounted at 90 degrees to the longitudinal axis of the fuselage. They can employ either bi-directional rotor blades or symmetrical airfoils with one blade being flipped during conversion. As yet there have not been any conventionally designed machines able to stop the rotor in forward flight and then restart it for vertical landing. The Herrick HV-2A Vertaplane of 1937 used a bi-directional rotor system that could be started in-flight from fixed-wing to autogiro mode, but not vice-versa. The US Naval Research Laboratory (NRL) Flip Rotor Concept is an example of an aircraft designed to use a conventional airfoil. The Bölkow/Stöckel P 109 Stopped Rotor concept of 1954-58 is another proposal that uses a transient conversion window.
The present invention seeks to overcome or substantially ameliorate at least some of the deficiencies of the prior art, or to at least provide an alternative.
It is to be understood that, if any prior art information is referred to herein, such reference does not constitute an admission that the information forms part of the common general knowledge in the art, in Australia or any other country.